This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 arising from an application entitled, METHOD FOR FORWARD POWER CONTROL IN CELLULAR SYSTEM, earlier filed in the Korean Industrial Property Office on Jul. 30, 1998, and there duly assigned Ser. No. 1998-30790.
1. Field of the Invention
The present invention relates to a cellular radio communication system, and more particularly, to a method for controlling the transmission power of a forward link to improve a call drop rate caused by a deterioration in the forward link in a code division multiple access (CDMA) system.
2. Description of the Related Art
In a cellular radio telecommunication system, the whole coverage area is divided into areas of a plurality of base stations (BSs) and a plurality of cells. The base stations are controlled by a mobile switching center MSC for allowing many subscribers to maintain a call while they are moving between the cells.
FIG. 1 illustrates a configuration of a conventional cellular radio communication system. It includes base stations (BSs) (200, 300 and 400) for providing a mobile communication services to a mobile station (MS) (100), and a mobile switching center (MSC) (500) for connecting the (BSs) (200, 300 and 400) to a public switched telephone network PSTN 600.
In the cellular system as illustrated in the FIG. 1, a forward link is directed from a base station to a mobile station, and a reverse link is directed from a mobile station to a base station. In a radio communication, a transmission power is controlled in order to suppress the transmission power. The power control is used to adjust the transmit power of the base station for each traffic channel. When the mobile station is affected by the signals traveling through propagation paths or other multi-path characteristic interference, the transmission power control is performed such that the received power levels of signals transmitted from the mobile stations and received by the base station, or the SIR (Signal-to-interference power ratio) associated with the received power levels are kept constant at the base station. By means of this forward power control, there is provided a saving of power consumption and a reduction of interference to other radio channels.
It will be appreciated by those skilled in the relevant art based on the detailed description provided herein that in a typical forward power control known in the art include a mobile station unit for measuring the quality of a communication signal received by the mobile station from a base station and a unit for producing a base station transmission power control data/forward power control signal, and the base station include a unit for measuring a receiving the forward power control signal, a unit for calculating a power level command based on the received forward power control signal, and a unit for producing a mobile station transmission power control data specifying a mobile station transmission power in accordance with the received.
Typically, a base station and a mobile station transmit a voice signal or a control signal in the frame unit of 20 ms. Each frame has error check bits known as cyclic redundancy checks (CRC). The mobile station checks for the error check bits in a forward frame received from the base station and determines whether the forward frame contains an error or not. At a data rate of 8 kbps, the mobile station periodically or when the number of non-proper frames received from the base station falls under a predetermined threshold value transmits a power measurement report message (PMRM) as a forward power control signal. The PMRM contains information about the quality of a forward link, i.e. the total number of properly received or non-properly received forward frames that the mobile station received from the base station. Using the PMRM, the base station adjusts its digital gain in the parameter to increase or decrease the transmit power at a predetermined level, which is indispensable in the CDMA scheme, as a reduction of an amount of interference with other radio channels can directly lead to an increase of a subscriber capacity. This procedure is referred as a Forward Power Control.
FIG. 2 illustrates a graph representing the forward power control as known in the prior art. As shown in the FIG. 2, a base station establishes a call with a mobile station and transmits a transmission power to the mobile station with a digital gain indicated as a nominal gain. Then, the base station starts to gradually decrease the transmission power. In other words, the base station gradually decreases the digital gain to reduce the transmission power at a decrement, referred as a xe2x80x9cslow down deltaxe2x80x9d in FIG. 2, starting from the peak of the nominal gain for every xe2x80x9cslow down time.xe2x80x9d
If a power measurement report message (PMRM) is not received from the mobile station during the first slow down time, the process of gradually decreasing the digital gain is repeated for a predetermined number of times, three times as shown in FIG. 2, the base station starts to rapidly decrease the digital gain to reduce the transmission power at another decrement, referred to as a xe2x80x9cfast down deltaxe2x80x9d in FIG. 2, for xe2x80x9cfast down time,xe2x80x9d repeatedly.
However, if the decreased digital gain is equal to a predetermined minimum gain, referred to as xe2x80x9cfloor one wayxe2x80x9d in FIG. 2., in either the slow down procedure or the fast down procedure, the base station ceases to further decrease the digital gain and maintains the digital gain at a predetermined minimum gain.
Meanwhile, the mobile station checks continually for the forward frames received from the base station and determines whether the forward frames have an error or not. Then, the mobile station informs the base station of the number of the forward frames and the number of forward bad frames via a power measurement report message (PMRM) periodically, or whenever the number of forward bad frames reaches at a predetermined threshold of bad frames (PWR13 REPT_THRESH).
If the PMRM is received by the base station via a reverse link during the slow down procedure or the fast down procedure, the base station calculates a frame error rate (FER) of the forward link, using the number of all forward frames and the number of bad forward frames as indicated in the PMRM.
If the calculated forward frame error rate exceeds the predetermined threshold of forward frame error rate (FER_THRESHOLD), the base station increases the digital gain of the base station to increase the transmission power by an increment, represented as xe2x80x9cbig up deltaxe2x80x9d at P10 in FIG. 2. Similarly, if the calculated forward frame error rate does not exceeds the FER_THRESHOLD, the base station increases the digital gain of transmission power by an increment, represented as xe2x80x9csmall up deltaxe2x80x9d at P20 in FIG. 2.
Accordingly, after the digital gain is increased either by the xe2x80x9cbig up deltaxe2x80x9d or the xe2x80x9csmall up delta,xe2x80x9d the base station decreases the digital gain according to the same slow down procedures and the fast down procedures described early. Whenever the power measurement report message is received from a mobile station, the base station adjusts the digital gain to increase or decrease the transmission power according to the newly received PMRM to carry out the forward power control.
Despite of performing such a forward power control, the quality of forward link is always susceptible to interference causing the mobile station to receive the forward frames having errors therein. As shown in FIG. 3, if 12 bad frames were received continuously for the duration of 240 ms (P1) after a call is established with a base station, the mobile station then stops outputting transmission power and enters into OFF mode.
If two good frames were not received within 5 seconds continuously, the mobile station releases the established call at P2 as shown in FIG. 3. However, if the mobile station in the OFF mode (between P1xcx9cP2) receives two good forward frames in sequence, the mobile station restarts outputting transmission power again.
As stated above, if the mobile station stops outputting transmission power and does not transmit any information via a reverse link, the base station cannot receive any power measurement report message (PMRM) including an information about frame error rate of the forward link. Then, the base station cannot receive any information about the quality of ongoing forward link, causing the base station to falsely believe that the forward link does not have an error, and continuously decrease the digital gain of transmission power to yield more deterioration in the forward link.
It is a primary object of the present invention to provide a forward power control employed in cellular system, and to make a base station to adjust the parameter to transmit power that enables the mobile station to restart outputting a Power Measurement Report Message (PMRM) in case when the mobile station has ceased to output the PMRM due to a deterioration in the forward link.
Another object of the present invention is to provide a forward power control method in cellular system so as to make the base station to rapidly decrease the transmission power after the mobile station has restarted to output the PMRM due to the power improvement in the forward link.
In one aspect of the inventions, a method for improving forward power control by in cellular mobile telecommunication system having a plurality of base stations and at least one mobile station, the method comprising the steps of:
a step (a) of establishing a call between the mobile station and the base station and transmitting a transmission power with a nominal gain as a digital gain via a forward link from the base station to the mobile station, and determining a signal quality on the forward link via one of a Power Measurement Report Message (PMRM) from the mobile station to the base station;
a step (b) of gradually decreasing the digital gain of the base station at a first slow-down power decrement every first slow-down time interval;
a step (c) of rapidly decreasing the digital gain of the base station at a first fast-down power decrement every first fast-down time interval if the PMRM is not received by the base station via a reverse link until the step (b) is repeated specific times;
a step (d) of changing the parameters of the forward power control by the base station if the PMRM is not received from the mobile station within the predetermined period and increasing the digital gain by a first power increment;
a step (e) of determining a frame error rate of the forward link if the PMRM is received by the base station from the mobile station responsive to the changed parameter of the forward power control, and comparing the determined frame error rate to a predetermined threshold value of frame error rate;
a step (f) of increasing the digital gain of the base station by the first power increment responsive to the changed parameter of the forward power control if the determined frame error rate is same or larger than the threshold value of frame error rate, gradually decreasing the digital gain of the base station at a second slow-down power decrement every second slow-down time interval, and rapidly decreasing the digital gain of the base station at a second fast-down power decrement every second fast-down time interval; and,
a step (g) of increasing the digital gain of the base station by a second power increment responsive to the changed parameter of the power control if the determined frame error rate is smaller than the threshold value of frame error rate, gradually decreasing the digital gain of the base station at a third slow-down power decrement every third slow-down time interval, and rapidly decreasing the digital gain of the base station at a third fast-down power decrement every third fast-down time interval, wherein the second power increment is smaller than the first power increment.
Another aspect of the present invention is to provide a method for improving forward power control in cellular mobile telecommunication system, the method comprising the steps of:
establishing a call between a mobile station and a base station and transmitting a transmission power by the base station via a forward link to the mobile station;
controlling forward power by decreasing a digital gain of the base station and increasing the digital gain according to an information about the quality of the forward link received from the mobile station;
checking forward frames received from the base station to the mobile station via the forward link and transmitting an information about the quality of the forward link as the checking result from the mobile station to the base station;
changing the parameters of the forward power control at the base station if an information about the quality of the forward link is not received by the mobile station within a predetermined period;
increasing the digital gain of the base station responsive to the changed parameter of the forward power control in order to receive the information from the mobile station; and,
decreasing the digital gain of the base station responsive to the changed parameters.